Optical information reproducing method, optical head device, and optical information processor

ABSTRACT

An optical information reproducing method of the present invention is used for an optical recording medium. The optical recording medium includes a recording layer containing information and a mask layer that is located close to the recording layer and includes a nonlinear optical material whose optical properties are changed in accordance with incident light intensity. The method includes irradiating the optical recording medium with convergent light that is polarized in a first direction and dividing reflected light from the optical recording medium into a first polarized component that is polarized in the first direction and a second polarized component that is polarized in a second direction perpendicular to the first direction. The second polarized component is used to detect a reproduction signal.

TECHNICAL FIELD

The present invention relates to an optical information reproducingmethod, an optical head device, and an optical information processingapparatus for optically reproducing information from optical disks oroptical cards.

BACKGROUND ART

In recent years, with the advance of the information society, there hasbeen a growing demand for a large-capacity external storage. For opticalinformation recording, an increase in density by reducing the size ofrecording pits has been restricted due to a diffraction limit thatdepends on the wavelength of light and the numerical aperture of anobjective lens.

Therefore, a so-called super-resolution recording/reproducing technologythat enables readout of a recording mark smaller than a focusing spotdiameter has been proposed as a means for achieving higher density. Forexample, JP 2000-348377 discloses a technique that uses near-field lightto perform recording/reproduction beyond the diffraction limit of light.

FIG. 8 shows the cross-sectional configuration of an optical recordingmedium used in super-resolution recording/reproduction. This opticalrecording medium 1 includes a transparent substrate 11 and a firstprotective layer 12, a mask layer 13, a second protective layer 14, arecording layer 15, and a third protective layer 16 that are formed onthe transparent substrate 11 in the indicated order. The first to thirdprotective layers 12, 14, and 16 are made of ZnS—SiO₂. The recordinglayer 15 is made of a phase change material (e.g., a multinary compoundsuch as GeSbTe). The mask layer 13 is made of silver oxide that isdecomposed into oxygen and silver by heat. When the optical recordingmedium 1 is irradiated with convergent light L1, a focusing spot isformed in the mask layer 13, and then the silver oxide is decomposedinto oxygen and silver to change the refractive index in ahigh-temperature portion of the focusing spot where the temperatureexceeds a given threshold value. Thus, an aperture 17 smaller than thefocusing spot diameter is formed in the mask layer 13 as a refractiveindex changing region. It is possible to write a recording mark 18 intothe recording layer 15 or read the recording mark 18 from the recordinglayer 15 using near-field light generated at the aperture 17. Therecording layer 15 is located at the position where the near-field lightgenerated in the mask layer 13 can reach, thereby achieving bothhigh-speed writing and high-speed reading.

A general optical head device has been used in the abovesuper-resolution recording/reproduction. FIG. 9 shows a general opticalhead device 101 when used for reproducing information from the opticalrecording medium 1. For convenience, the lateral direction of the sheetof this drawing is identified as an X-direction, the vertical directionfrom the sheet surface is identified as a Y-direction, and thelongitudinal direction of the sheet is identified as a Z-direction.

A semiconductor laser 102 (a radiation light source) radiates linearlypolarized light that is polarized in the X-direction. The light emittedfrom the semiconductor laser 102 enters a polarization beam splitter103. The polarization beam splitter 103 has the functions oftransmitting all the light polarized in the X-direction and reflectingall the light polarized in the Y-direction. The light passing throughthe polarization beam splitter 103 is converted into parallel light by acollimator lens 104, then converted into circularly polarized light by aquarter-wave plate 105, and focused to the inside of the opticalrecording medium 1 by an objective lens 106. The light reflected fromthe optical recording medium 1 again passes through the objective lens106 and the quarter-wave plate 105, and thus is converted into linearlypolarized light that is polarized in the Y-direction. This linearlypolarized light further passes through the collimator lens 104 andenters the polarization beam splitter 103. The light entering thepolarization beam splitter 103 is polarized in the Y-direction, andtherefore is reflected by the polarization beam splitter 103. Thereflected light passes through a cylindrical lens 107 or the like sothat the wavefront is controlled to detect a servo signal. Subsequently,a photodetector 108 detects a reproduction signal and a servo signal.The quarter-wave plate 105 and the polarization beam splitter 103 areused to improve the light utilization efficiency. Even if thequarter-wave plate 105 is not provided, and the polarization beamsplitter 103 is replaced with a non-polarization beam splitter,information can be recorded/reproduced.

When optical information is reproduced in the above manner, a generallaser beam includes a noise component. For normal reproduction (ratherthan the super-resolution reproduction), the noise component is not aproblem because the degree of signal modulation is sufficient. In thecase of super-resolution reproduction, however, a recording mark smallerthan the spot diameter of the laser beam should be read, so that thedegree of modulation of a reproduction signal is reduced significantly.Therefore, the effect of the noise component of the laser beam cannot beignored, which may lead to S/N degradation.

DISCLOSURE OF INVENTION

An optical information reproducing method of the present invention isused for an optical recording medium. The optical recording mediumincludes a recording layer containing information and a mask layer thatis located close to the recording layer and includes a nonlinear opticalmaterial whose optical properties are changed in accordance withincident light intensity. The method includes the following: irradiatingthe optical recording medium with convergent light that is polarized ina first direction; forming an optical property changing region in aportion of a region of the mask layer that is exposed to the convergentlight; irradiating the recording layer with light passing through theoptical property changing region; dividing reflected light from theoptical recording medium into a first polarized component that ispolarized in the first direction and a second polarized component thatis polarized in a second direction perpendicular to the first direction;and detecting a reproduction signal by using the second polarizedcomponent.

An optical head device of the present invention is used for reproducinginformation from an optical recording medium. The optical recordingmedium includes a recording layer containing the information and a masklayer that is located close to the recording layer and includes anonlinear optical material whose optical properties are changed inaccordance with incident light intensity. The optical head deviceincludes the following: a radiation light source for radiating lightthat is polarized in a first direction; a focusing optical system forconverging the light emitted from the radiation light source on theoptical recording medium to form a tiny spot; a polarization separationoptical system for dividing reflected light from the optical recordingmedium into a first polarized component that is polarized in the firstdirection and a second polarized component that is polarized in a seconddirection perpendicular to the first direction; and a photodetector fordetecting a reproduction signal by receiving the second polarizedcomponent separated by the polarization separation optical system.

An optical information processing apparatus of the present inventionincludes an optical recording medium and an optical head device. Theoptical recording medium includes a recording layer containinginformation and a mask layer that is located close to the recordinglayer and includes a nonlinear optical material whose optical propertiesare changed in accordance with incident light intensity. The opticalhead device includes the following: a radiation light source forradiating linearly polarized light that is polarized in a firstdirection; a focusing optical system for converging the light emittedfrom the radiation light source on the optical recording medium to forma tiny spot; a polarization separation optical system for dividingreflected light from the optical recording medium into a first polarizedcomponent that is polarized in the first direction and a secondpolarized component that is polarized in a second directionperpendicular to the first direction; and a photodetector for detectinga reproduction signal by receiving the second polarized componentseparated by the polarization separation optical system.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view showing an example of the configurationof an optical recording medium used for super-resolutionrecording/reproduction.

FIG. 2 is a cross-sectional view showing how to perform super-resolutionrecording/reproduction with respect to an optical recording medium.

FIG. 3 is a schematic diagram showing the configuration of an opticalhead device in Embodiment 1 of the present invention.

FIG. 4 is a schematic diagram showing the configuration of an opticalhead device in Embodiment 2 of the present invention.

FIG. 5 is a schematic diagram showing the configuration of an opticalhead device in Embodiment 3 of the present invention.

FIG. 6 is a schematic diagram showing the configuration of an opticalhead device in Embodiment 4 of the present invention.

FIG. 7 is a schematic diagram showing the configuration of an opticalinformation processing apparatus in Embodiment 5 of the presentinvention.

FIG. 8 is a cross-sectional view showing how to perform super-resolutionrecording/reproduction with respect to an optical recording medium.

FIG. 9 is a schematic diagram showing the configuration of aconventional optical head device.

BEST MODE FOR CARRYING OUT THE INVENTION

An optical information reproducing method of the present invention isused for an optical recording medium. The optical recording mediumincludes a recording layer containing information and a mask layer thatis located close to the recording layer and includes a nonlinear opticalmaterial whose optical properties are changed in accordance withincident light intensity. The method includes the following: irradiatingthe optical recording medium with convergent light that is polarized ina first direction; forming an optical property changing region in aportion of a region of the mask layer that is exposed to the convergentlight; irradiating the recording layer with light passing through theoptical property changing region; dividing reflected light from theoptical recording medium into a first polarized component that ispolarized in the first direction and a second polarized component thatis polarized in a second direction perpendicular to the first direction;and detecting a reproduction signal by using the second polarizedcomponent. The reflected light from the optical recording medium used inthe optical information reproducing method of the present inventionincludes light reflected from the recording layer and light reflectedfrom the mask layer. The light reflected from the recording layerrepresents information (including reproduction information) recorded onthe recording layer and has a high degree of modulation. The lightreflected from the mask layer includes no reproduction information.There is a difference in polarization state between them. Specifically,the light reflected from the mask layer has the same polarization stateas the convergent light for irradiation. In contrast, the lightreflected from the recording layer is scattered in the optical propertychanging region of the mask layer, and also includes a polarizedcomponent perpendicular to the polarization direction of the convergentlight. The optical information reproducing method of the presentinvention allows the component (the second polarized component), whichis polarized in the direction (the second direction) perpendicular tothe polarization direction (the first direction) of the irradiationlight and is included in light that represents reproduction informationand has a high degree of modulation, to be taken out of the reflectedlight from the optical recording medium and used to detect areproduction signal. Thus, a reproduction signal with a high degree ofmodulation can be obtained, and S/N can be improved in super-resolutionreproduction.

In the optical recording medium used in the optical informationreproducing method of the present invention, the nonlinear opticalmaterial of the mask layer may be at least one selected from antimony(Sb), silver oxide, a semiconductor, chalcogenide glass, and athermochromic material. As the semiconductor, e.g., a semiconductorincluding elements such as silicon (Si) and germanium (Ge) can be used.The thermochromic material is a material that reversibly changes incolor with temperature.

In the optical information reproducing method of the present invention,it is desirable that a servo signal is detected by using the firstpolarized component that is separated from the reflected light from theoptical recording medium.

An optical head device of the present invention is used for reproducinginformation from an optical recording medium. The optical recordingmedium includes a recording layer containing the information and a masklayer that is located close to the recording layer and includes anonlinear optical material whose optical properties are changed inaccordance with incident light intensity. The optical head deviceincludes the following: a radiation light source for radiating lightthat is polarized in a first direction; a focusing optical system forconverging the light emitted from the radiation light source on theoptical recording medium to form a tiny spot; a polarization separationoptical system for dividing reflected light from the optical recordingmedium into a first polarized component that is polarized in the firstdirection and a second polarized component that is polarized in a seconddirection perpendicular to the first direction; and a photodetector fordetecting a reproduction signal by receiving the second polarizedcomponent separated by the polarization separation optical system. Inthis case, the reflected light from the optical recording mediumincludes light that represents information recorded on the recordinglayer and has a high degree of modulation (the light reflected from therecording layer) and light that includes no reproduction information(the light reflected from the mask layer), as described above. However,the optical head device of the present invention can separate only thelight with a high degree of modulation from the reflected light of theoptical recording medium and detect a reproduction signal. Thus, S/N canbe improved in super-resolution reproduction.

The polarization separation optical system of the optical head device ofthe present invention may include, e.g., a polarization beam splitterthat transmits all the first polarized component and reflects all thesecond polarized component or a polarization beam splitter that reflectsall the first polarized component and transmits all the second polarizedcomponent. Alternatively, the polarization separation optical system mayinclude either a diffraction element that diffracts all the firstpolarized component and transmits all the second polarized component ora polarizing prism that separates the first polarized component from thesecond polarized component.

It is desirable that the optical head device of the present inventionfurther includes a servo signal photodetector that detects a servosignal by receiving the first polarized component separated by thepolarization separation optical system.

An optical information processing apparatus of the present inventionincludes an optical recording medium and an optical head device. Theoptical recording medium includes a recording layer containinginformation and a mask layer that is located close to the recordinglayer and includes a nonlinear optical material whose optical propertiesare changed in accordance with incident light intensity. The opticalhead device includes the following: a radiation light source forradiating linearly polarized light that is polarized in a firstdirection; a focusing optical system for converging the light emittedfrom the radiation light source on the optical recording medium to forma tiny spot; a polarization separation optical system for dividingreflected light from the optical recording medium into a first polarizedcomponent that is polarized in the first direction and a secondpolarized component that is polarized in a second directionperpendicular to the first direction; and a photodetector for detectinga reproduction signal by receiving the second polarized componentseparated by the polarization separation optical system. The reflectedlight from the optical recording medium in the optical informationprocessing apparatus of the present invention includes light thatrepresents information recorded on the recording layer and has a highdegree of modulation (the light reflected from the recording layer) andlight that includes no reproduction information (the light reflectedfrom the mask layer). However, the optical information processingapparatus of the present invention can separate only the light with ahigh degree of modulation from the reflected light of the opticalrecording medium and detect a reproduction signal. Thus, S/N can beimproved in super-resolution reproduction.

In the optical information processing apparatus of the presentinvention, the nonlinear optical material of the mask layer of theoptical recording medium may be at least one selected from Sb, silveroxide, a semiconductor, chalcogenide glass, and a thermochromicmaterial. As the semiconductor, e.g., a semiconductor including elementssuch as Si and Ge can be used. The thermochromic material is a materialthat reversibly changes in color with temperature.

In the optical information processing apparatus of the presentinvention, the polarization separation optical system of the opticalhead device may include, e.g., a polarization beam splitter thattransmits all the first polarized component and reflects all the secondpolarized component or a polarization beam splitter that reflects allthe first polarized component and transmits all the second polarizedcomponent. Alternatively, the polarization separation optical system mayinclude either a diffraction element that diffracts all the firstpolarized component and transmits all the second polarized component ora polarizing prism that separates the first polarized component from thesecond polarized component.

In the optical information processing apparatus of the presentinvention, it is desirable that the optical head device further includesa servo signal photodetector that detects a servo signal by receivingthe first polarized component separated by the polarization separationoptical system.

Hereinafter, embodiments of the present invention will be described withreference to the drawings.

Embodiment 1

First, the configuration of an optical recording medium in thisembodiment will be described by referring to FIG. 1. An opticalrecording medium 1 of this embodiment includes a transparent substrate11, a first protective layer 12, a mask layer 13, a second protectivelayer 14, a recording layer 15, and a third protective layer 16 in theindicated order from the incident side of a laser beam forrecording/reproducing information.

The transparent substrate 11 is made of a material that is transparentor almost transparent to the laser beam, and transmits or substantiallytransmits the laser beam. Guide grooves for the laser beam preferablyare formed in the surface of the transparent substrate 11. FIG. 1 showsthe shape of the optical recording medium 1 provided with the guidegrooves.

The mask layer 13 includes a nonlinear optical material whose opticalproperties are changed in accordance with incident light intensity. Thisembodiment uses a Sb film having a thickness of 20 nm.

The recording layer 15 is provided to record information and made of amaterial whose state is changed by light or heat generation resultingfrom the absorption of light. This embodiment uses a Ge₃₀Sb₁₅Te₅₅ filmhaving a thickness of 15 nm.

The first to third protective layers 12, 14, and 16 are provided mainlyto protect the mask layer 13 and the recording layer 15 and to adjustthe optical properties so that the recording layer 13 can absorb lighteffectively. In this embodiment, a material obtained by mixing ZnS with20 mol % of SiO₂ is used, and the thicknesses of the first, second, andthird protective layers 12, 14, and 16 are 40 nm, 20 nm, and 130 nm,respectively.

Next, an optical information reproducing method in this embodiment willbe described by referring to FIG. 2. To record/reproduce informationwith respect to the optical recording medium 1, the optical recordingmedium 1 is irradiated with a linearly polarized laser beam L1 that isconvergent light focused by an objective lens (not shown). Thisirradiation light is diffracted by the aperture of the objective lensand has an intensity distribution with a shape close to Gaussiandistribution in which the radius of a portion where the intensity is1/e² with respect to the central portion is 0.41 λ/NA (λ is thewavelength of a laser beam for irradiation, and NA is the numericalaperture of an objective lens).

In the Sb film (the mask layer 13), the temperature rises in the centralportion of a region exposed to the laser beam L1, and the opticalproperties are changed. Consequently, a region (an optical propertychanging region) that changes in optical properties is formed in aportion of the region of the mask layer 13 that is exposed to the laserbeam L1. In this portion, the light intensity of the laser beam L1 islarger than a predetermined value. The optical property changing regionhas a high transmittance with respect to the laser beam L1 and serves asan aperture 17. The size of the aperture 17 is smaller than the spot ofthe laser beam L1 falling on the mask layer 13.

By using a laser beam L2 that is transmitted through the aperture 17thus formed in the mask layer 13, information is recorded on therecording layer 15 and the information is reproduced from the recordinglayer 15. The recordable/reproducible minimum mark size is determined bythe size of the aperture 17 and independent of the diffraction limit.Accordingly, information of not more than the diffraction limit can berecorded/reproduced, i.e., so-called super-resolutionrecording/reproduction can be performed.

The information recorded on the recording layer 15 is reproduced bydetecting reflected light L3 of the laser beam L1. The reflected lightL3 includes a component of light in the central portion of the spot(i.e., a central light component) L3 a (not shown) and a component oflight on the periphery of the spot (i.e., a peripheral light component)L3 b (not shown). The central light component L3 a is light thatrepresents the information recorded on the recording layer 15 and has ahigh degree of modulation. The peripheral light component L3 b is lightthat is reflected from the mask layer 13 before reaching the recordinglayer 15 and includes no reproduction information. For this reason, whena reproduction signal is detected from the whole of the reflected lightL3, the degree of signal modulation is reduced, and S/N is degraded.

Therefore, the optical information reproducing method of the presentinvention allows the central light component L3 a to be taken out of thereflected light L3 and detected as a reproduction signal, thusperforming favorable reproduction with an improved degree of signalmodulation. In the optical information reproducing method of the presentinvention, a difference in polarization between the central lightcomponent L3 a and the peripheral light component L3 b is utilized toremove the peripheral light component L3 b from the reflected light L3.Since the peripheral light component L3 b is reflected from the masklayer 13, the polarization direction is the same as that of the laserbeam L1. The central light component L3 a is scattered while passingthrough the aperture 17, and also includes a polarized componentperpendicular to the laser beam L1. Therefore, when the polarizedcomponent perpendicular to the laser beam L1 is taken out of thereflected light L3 and detected, a reproduction signal having a highdegree of modulation can be obtained. This makes it possible to reducethe effect of noise and improve S/N.

Next, an example of an optical head device used in the opticalinformation reproducing method of the present invention will bedescribed. FIG. 3 shows an optical head device 2 of this embodiment whenused for reproducing information from the optical recording medium 1.For convenience, the lateral direction of the sheet of this drawing isidentified as an X-direction, the vertical direction from the sheetsurface is identified as a Y-direction, and the longitudinal directionof the sheet is identified as a Z-direction.

A semiconductor laser 21 (a radiation light source) radiates linearlypolarized light that is polarized in the X-direction (a firstdirection). A laser beam L1 emitted from the semiconductor laser 21enters a polarization beam splitter 22. The polarization beam splitter22 has the functions of dividing the light polarized in the X-directioninto transmission and reflection at a desired ratio (e.g., atransmittance of 80% and a reflectance of 20% in this embodiment) andreflecting all the light polarized in the Y-direction (a seconddirection). The laser beam L1 passing through the polarization beamsplitter 22 is converted into parallel light by a collimator lens 23 andfocused to the inside of the optical recording medium 1 by an objectivelens 24. The optical recording medium 1 has the structure as shown inFIGS. 1 and 2. Reflected light L3 that is reflected from the opticalrecording medium 1 again passes through the objective lens 24 and thecollimator lens 23, and then enters the polarization beam splitter 22,where 20% of the light polarized in the X-direction is reflected and allthe light polarized in the Y-direction is reflected. The light reflectedfrom the polarization beam splitter 22 further enters a polarizationbeam splitter 25 that serves as a polarization separation opticalsystem. The polarization beam splitter 25 has the functions oftransmitting all the light polarized in the X-direction and reflectingall the light polarized in the Y-direction. Therefore, the polarizationbeam splitter 25 can separate the component (a first polarizedcomponent) that is polarized in the X-direction from the component (asecond polarized component) that is polarized in the Y-direction.

A photodetector 28 receives the component polarized in the Y-directionand detects a reproduction signal. The light polarized in theX-direction that has passed through the polarization beam splitter 25enters a hologram element 26, where the wavefront is transformed so asto detect a servo signal. Then, the light enters a photodetector 27 thatserves as a servo signal photodetector. The photodetector 27 includes aplurality of light-receiving regions and can detect servo signals fromthe signals in each of the light-receiving regions.

As described above, the optical head device 2 of this embodiment canseparate the component that represents recorded information from thereflected light and perform super-resolution reproduction with improvedS/N.

In this embodiment, the polarization beam splitter that transmits allthe light polarized in the X-direction and reflects all the lightpolarized in the Y-direction is used as a polarization separationoptical system. Needless to say, it is also possible to use apolarization beam splitter that reflects all the light polarized in theX-direction and transmits all the light polarized in the Y-direction.

Embodiment 2

The following is an explanation of another embodiment of an optical headdevice of the present invention. FIG. 4 shows an optical head device 3of this embodiment when used for reproducing information from theoptical recording medium 1. For convenience, the lateral direction ofthe sheet of this drawing is identified as an X-direction, the verticaldirection from the sheet surface is identified as a Y-direction, and thelongitudinal direction of the sheet is identified as a Z-direction.

A semiconductor laser 31 (a radiation light source) radiates linearlypolarized light that is polarized in the X-direction (a firstdirection). A laser beam L1 emitted from the semiconductor laser 31enters a polarization beam splitter 32. The polarization beam splitter32 has the functions of transmitting all the light polarized in theX-direction and reflecting all the light polarized in the Y-direction (asecond direction). The laser beam L1 passing through the polarizationbeam splitter 32 is converted into parallel light by a collimator lens33 and focused to the inside of the optical recording medium 1 by anobjective lens 34. The optical recording medium 1 has the structure asshown in FIGS. 1 and 2. Reflected light L3 that is reflected from theoptical recording medium 1 again passes through the objective lens 34and the collimator lens 33, and then enters the polarization beamsplitter 32, where all the light polarized in the X-direction (a firstpolarized component) is transmitted and all the light polarized in theY-direction (a second polarized component) is reflected. In other words,the polarization beam splitter 32 functions as a polarization separationoptical system that separates the first polarized component and thesecond polarized component from the reflected light.

A photodetector 35 receives the component polarized in the Y-directionand detects a reproduction signal. The light polarized in theX-direction that has been separated by the polarization beam splitter 32enters a hologram element 36, where the wavefront is transformed bydiffraction of the hologram element 36 so as to detect a servo signal.Then, the light enters a photodetector 37 that serves as a servo signalphotodetector. The photodetector 37 includes a plurality oflight-receiving regions and can detect servo signals from the signals ineach of the light-receiving regions. In this configuration, thesemiconductor laser 31 can be located close to the photodetector 37 fordetecting a servo signal, thus improving stability. Moreover, thesemiconductor laser 31, the photodetector 37, and the hologram element36 are integrated in the same package, so that the optical head devicecan have higher stability.

As described above, the optical head device 3 of this embodiment canseparate the component that represents recorded information from thereflected light and perform super-resolution reproduction with improvedS/N.

Embodiment 3

The following is an explanation of another embodiment of an optical headdevice of the present invention. FIG. 5 shows an optical head device 4of this embodiment when used for reproducing information from theoptical recording medium 1. For convenience, the lateral direction ofthe sheet of this drawing is identified as an X-direction, the verticaldirection from the sheet surface is identified as a Y-direction, and thelongitudinal direction of the sheet is identified as a Z-direction.

A semiconductor laser 41 (a radiation light source) radiates linearlypolarized light that is polarized in the X-direction (a firstdirection). A laser beam L1 emitted from the semiconductor laser 41enters a polarization beam splitter 42. The polarization beam splitter42 has the functions of dividing the light polarized in the X-directioninto transmission and reflection at a desired ratio (e.g., atransmittance of 80% and a reflectance of 20% in this embodiment) andreflecting all the light polarized in the Y-direction (a seconddirection). The laser beam L1 passing through the polarization beamsplitter 42 is converted into parallel light by a collimator lens 43 andfocused to the inside of the optical recording medium 1 by an objectivelens 44. The optical recording medium 1 has the structure as shown inFIGS. 1 and 2. Reflected light L3 that is reflected from the opticalrecording medium 1 again passes through the objective lens 44 and thecollimator lens 43, and then enters the polarization beam splitter 42,where 20% of the light polarized in the X-direction is reflected and allthe light polarized in the Y-direction is reflected. The light reflectedfrom the polarization beam splitter 42 further enters a polarizationanisotropic hologram 45 that serves as a polarization separation opticalsystem. The polarization anisotropic hologram 45 is a diffractionelement that diffracts the component polarized in the X-direction (afirst polarized component) at a desired wavefront so as to detect aservo signal and transmits the component polarized in the Y-direction (asecond polarized component). Therefore, the polarization anisotropichologram 45 can separate the two polarized components.

The component polarized in the Y-direction that has been transmittedthrough the polarization anisotropic hologram 45 is detected in alight-receiving region of a photodetector 46 and converted into areproduction signal. The component polarized in the X-direction that hasbeen diffracted by the polarization anisotropic hologram 45 is detectedin a plurality of light-receiving regions of the photodetector 46 andconverted into servo signals. In this configuration, the photodetector46 is used for a reproduction signal as well as a servo signal.

As described above, the optical head device 4 of this embodiment canseparate the component that represents recorded information from thereflected light and perform super-resolution reproduction with improvedS/N.

Embodiment 4

The following is an explanation of another embodiment of an optical headdevice of the present invention. FIG. 6 shows an optical head device 5of this embodiment when used for reproducing information from theoptical recording medium 1. For convenience, the lateral direction ofthe sheet of this drawing is identified as an X-direction, the verticaldirection from the sheet surface is identified as a Y-direction, and thelongitudinal direction of the sheet is identified as a Z-direction.

A semiconductor laser 51 (a radiation light source) radiates linearlypolarized light that is polarized in the X-direction (a firstdirection). A laser beam L1 emitted from the semiconductor laser 51enters a polarization beam splitter 52. The polarization beam splitter52 has the functions of dividing the light polarized in the X-directioninto transmission and reflection at a desired ratio (e.g., atransmittance of 80% and a reflectance of 20% in this embodiment) andreflecting all the light polarized in the Y-direction (a seconddirection). The laser beam L1 passing through the polarization beamsplitter 52 is converted into parallel light by a collimator lens 53 andfocused to the inside of the optical recording medium 1 by an objectivelens 54. The optical recording medium 1 has the structure as shown inFIGS. 1 and 2. Reflected light L3 that is reflected from the opticalrecording medium 1 again passes through the objective lens 54 and thecollimator lens 53, and then enters the polarization beam splitter 52,where 20% of the light polarized in the X-direction is reflected and allthe light polarized in the Y-direction is reflected. The light reflectedfrom the polarization beam splitter 52 further enters a Wollaston prism(a polarizing prism) 55 that serves as a polarization separation opticalsystem, and then is divided into the component polarized in theX-direction (a first polarized component) and the component polarized inthe Y-direction (a second polarized component).

The component polarized in the Y-direction that has been separated bythe Wollaston prism 55 is detected in a light-receiving region of aphotodetector 57 and converted into a reproduction signal. The componentpolarized in the X-direction that has been separated by the Wollastonprism 55 is detected in a plurality of light-receiving regions of thephotodetector 57 and converted into servo signals. In thisconfiguration, the photodetector 57 is used for a reproduction signal aswell as a servo signal.

As described above, the optical head device 5 of this embodiment canseparate the component that represents recorded information from thereflected light and perform super-resolution reproduction with improvedS/N.

Embodiment 5

An example of an optical information processing apparatus of the presentinvention will be described below.

FIG. 7 shows the schematic configuration of an optical informationprocessing apparatus of this embodiment. An optical disk 61 is adisk-shaped optical recording medium and is rotated by a rotatingmechanism 62. The optical disk 61 has the same configuration as theoptical recording medium 1 used in Embodiments 1 to 4. An optical headdevice 63 may be any one of the optical head devices 2 to 5 inEmbodiments 1 to 4 and has a fine adjustment means for an objectivelens. A drive 64 moves the optical head device 63 to a track of theoptical disk that contains desired information. The optical head device63 transmits a focus error signal or tracking error signal to anelectric circuit 66 in accordance with the positional relationshipbetween the optical head device 63 and the optical disk 61. In responseto these signals, the electric circuit 66 transmits a signal for finelyadjusting the objective lens to the optical head device 64. Uponreceiving the signal, the optical head device 64 performs focus servoand tracking servo on the optical disk 61 so that information is read,written, or erased with respect to the optical disk 61. The electriccircuit 66 also has the functions of adjusting the intensity of a laserbeam to optimize a reproduction signal and controlling the size of anaperture formed in the mask layer of the optical disk 61.

This configuration allows the optical information processing apparatusto perform super-resolution reproduction with improved S/N.

In Embodiment 1 to 4, a Sb film is used as the mask layer 13 of theoptical recording medium 1. However, the mask layer 13 is not limitedthereto, and may be made of a material whose optical properties arechanged reversibly in accordance with light intensity. For example,silver oxide, a semiconductor (including elements such as Si and Ge),chalcogenide glass, and a thermochromic material also can be used.Moreover, Ge₃₀Sb₁₅Te₅₅ is used as the recording layer 15. However, therecording layer 15 is not limited thereto, and may be made of a material(e.g., various organic dyes or chalcogenide glass) that canrecord/reproduce information by light. The recording layer 13 can be areflection film on which information is recorded beforehand inconcave-concave form.

1. An optical information reproducing method for reproducing informationfrom an optical recording medium, the optical recording mediumcomprising: a recording layer containing the information; and a masklayer that is located close to the recording layer and includes anonlinear optical material whose optical properties are changed inaccordance with incident light intensity, the method comprising:irradiating the optical recording medium with convergent light that ispolarized in a first direction; forming an optical property changingregion in a portion of a region of the mask layer that is exposed to theconvergent light; irradiating the recording layer with light passingthrough the optical property changing region; dividing reflected lightfrom the optical recording medium into a first polarized component thatis polarized in the first direction and a second polarized componentthat is polarized in a second direction perpendicular to the firstdirection; and detecting a reproduction signal by using the secondpolarized component.
 2. The method according to claim 1, wherein thenonlinear optical material is at least one selected from the groupconsisting of antimony, silver oxide, a semiconductor, chalcogenideglass, and a thermochromic material.
 3. The method according to claim 1,wherein a servo signal is detected by using the first polarizedcomponent that is separated from the reflected light from the opticalrecording medium.
 4. An optical head device for reproducing informationfrom an optical recording medium, the optical recording mediumcomprising: a recording layer containing the information; and a masklayer that is located close to the recording layer and includes anonlinear optical material whose optical properties are changed inaccordance with incident light intensity, the optical head devicecomprising: a radiation light source for radiating light that ispolarized in a first direction; a focusing optical system for convergingthe light emitted from the radiation light source on the opticalrecording medium to form a tiny spot; a polarization separation opticalsystem for dividing reflected light from the optical recording mediuminto a first polarized component that is polarized in the firstdirection and a second polarized component that is polarized in a seconddirection perpendicular to the first direction; and a photodetector fordetecting a reproduction signal by receiving the second polarizedcomponent separated by the polarization separation optical system. 5.The optical head device according to claim 4, wherein the polarizationseparation optical system comprises a polarization beam splitter thattransmits all the first polarized component and reflects all the secondpolarized component or a polarization beam splitter that reflects allthe first polarized component and transmits all the second polarizedcomponent.
 6. The optical head device according to claim 4, wherein thepolarization separation optical system comprises a diffraction elementthat diffracts all the first polarized component and transmits all thesecond polarized component.
 7. The optical head device according toclaim 4, wherein the polarization separation optical system comprises apolarizing prism that separates the first polarized component from thesecond polarized component.
 8. The optical head device according toclaim 4, further comprising a servo signal photodetector that detects aservo signal by receiving the first polarized component separated by thepolarization separation optical system.
 9. An optical informationprocessing apparatus comprising: an optical recording medium; and anoptical head device, the optical recording medium comprising: arecording layer containing information; and a mask layer that is locatedclose to the recording layer and includes a nonlinear optical materialwhose optical properties are changed in accordance with incident lightintensity, the optical head device comprising: a radiation light sourcefor radiating linearly polarized light that is polarized in a firstdirection; a focusing optical system for converging the light emittedfrom the radiation light source on the optical recording medium to forma tiny spot; a polarization separation optical system for dividingreflected light from the optical recording medium into a first polarizedcomponent that is polarized in the first direction and a secondpolarized component that is polarized in a second directionperpendicular to the first direction; and a photodetector for detectinga reproduction signal by receiving the second polarized componentseparated by the polarization separation optical system.
 10. The opticalinformation processing apparatus according to claim 9, wherein thenonlinear optical material is at least one selected from the groupconsisting of antimony, silver oxide, a semiconductor, chalcogenideglass, and a thermochromic material.
 11. The optical informationprocessing apparatus according to claim 9, wherein the polarizationseparation optical system comprises a polarization beam splitter thattransmits all the first polarized component and reflects all the secondpolarized component or a polarization beam splitter that reflects allthe first polarized component and transmits all the second polarizedcomponent.
 12. The optical information processing apparatus according toclaim 9, wherein the polarization separation optical system comprises adiffraction element that diffracts all the first polarized component andtransmits all the second polarized component.
 13. The opticalinformation processing apparatus according to claim 9, wherein thepolarization separation optical system comprises a polarizing prism thatseparates the first polarized component from the second polarizedcomponent.
 14. The optical information processing apparatus according toclaim 9, further comprising a photodetector that detects a servo signalby receiving the first polarized component separated by the polarizationseparation optical system.